1. Field of the Invention
The present invention relates to the configuration of an optical modulator used in the field of optical communications as a phase modulator, intensity modulator or polarization modulator. It particularly relates to the configuration of a resonance type optical modulator using a symmetric or asymmetric electrode having high modulation efficiency even with a low input voltage.
2. Description of the Prior Art
An optical modulator is a device that converts electrical signal information, such as intensity, phase and frequency, into optical carrier information that is then output. Modulation methods include the direct method in which a source laser is directly modulated, and the method in which the source light wave is modulated by an external modulator. The direct modulation method can be realized with a simple system configuration, while the method using an external modulator provides high-quality modulation. For communications involving very high speeds and long distances, modulation is usually carried out using an external modulator.
The external modulators used include ones that utilize the electro-optical effect or the semiconductor electro-absorption effect. The former relates to the present invention and, as such, is described in further detail below.
LiNbO3, a substance that exhibits the electro-optical effect, is extensively used for optical modulators. The modulation utilizes changes in the refractive index based on the Pockels effect that is the primary electro-optical effect and a characteristic of LiNbO3. It is well known that in the modulation, an electric field is applied parallel or perpendicular to the surface of the LiNbO3 substrate. Which it is depends on the crystal orientation of the LiNbO3 substrate. The field is applied parallel to an x-cut substrate and perpendicular to a z-cut substrate.
Another property of LiNbO3 is that its refractive index can be readily adjusted by thermally diffusing a substance, such as titanium, into the substrate. This is often used to form an optical waveguide in the LiNbO3 substrate and integrate a plurality of the devices.
Modulators that use a LiNbO3 substrate include standing wave type optical modulators and resonance type optical modulators. In a standing wave type optical modulator the optical waves are guided in the same direction as the electric signals, with the light wave being modulated in the waveguides. Such modulators have a wide bandwidth ranging from direct-current signals to microwave signals, but to be effective require a high driving current. On the other hand, since resonance type optical modulators use resonance for the modulation, the bandwidth is limited to a narrow band in the microwave region, but modulation is highly efficient. This makes them effective for applications that require high-efficiency operation in a narrow band, such as polarization scramblers and multiplexers.
The present invention relates to the configuration of this type of resonance type optical modulator, which will therefore now be described in the following. FIGS. 1A and 1B illustrate a prior-art resonance type optical modulator (xe2x80x9cStudy of 10 GHz resonance type LiNbO3 optical modulator,xe2x80x9d Oikawa and thee others, Report No. C-3-25, p. 204, of the collected papers of the general conference of the Institute of Electronics, Information and Communication Engineers (2000)). FIG. 1A shows a z-cut type modulator comprised of an optical waveguide, an electrode (modulation electrode) above the optical waveguide and a capacitor for impedance matching. The electrode is of a size that resonates at a microwave frequency input. The modulator shown in FIG. 1B comprises a modulation electrode and an open stub for impedance matching. This modulator is easier to manufacture than the modulator of FIG. 1A.
In the modulator of FIG. 1B, the modulation electrode and the stub are formed of layers of the same metal, which makes the modulator easier to fabricate. However, in addition to being provided on the microwave signal input side, a common level line layer is also provided on the other side of the optical path, and at a position that circumvents the modulation electrode. The configuration therefore has the drawback of being a common electrode while also trying to be a common level structure. Moreover, another drawback is that the effective modulation field strength applied to the optical path is substantially half that compared to when the common level line layer is provided only on the microwave signal input side.
In view of the above shortcomings, the present invention has been proposed, and an object thereof is to provide a resonance type optical modulator using a symmetric or asymmetric electrode that is easy to manufacture and provides highly effective modulation even with a low input voltage, having a configuration that does not give rise to loss of the effective modulation field strength applied to the optical path.
In the description of the invention given below, an open stub means that the end of a microwave asymmetric coplanar waveguide that is not the signal-supply end is open-ended, and a short stub means that the aforementioned end is short-ended.
The first point of the present invention to attain the above object relates to provide a resonance type optical modulator comprising an optical path having electro-optical effect characteristics, a modulation electrode formed along the optical path for applying an electric field to the optical path, a common electrode formed in opposition to the modulation electrode, a feeding line that is electromagnetically connected to the modulation electrode, and stubs connected to the feeding line; the feeding line, stubs and common electrode being provided on one side of a region that is divided by the modulation electrode.
A coaxial cable is generally used to feed the signal to the modulator from an external signal source, and since the feed has to be made to a very small region while performing impedance matching, the second point of the invention has a feature that the feeding line includes a tapered transformer in addition to the first point.
There can be one stub or a plurality of stubs. When there is a plurality of stubs, the design is facilitated if the stubs are positioned with bilateral symmetry. Therefore, the third point of the present invention relates to a resonance type optical modulator comprising an even number of stubs that are located symmetrically with respect to the feeding line in addition to the first or second point.
In the resonance type optical modulator according to the first point of the present invention, moreover, it is desirable for the stub to be positioned at the junction between the modulation electrode and the feeding line. Therefore the fourth point of the present invention relates to a modulator comprising the modulation electrode and feeding line intersecting at right angles, in addition to the first point, with the stub being located at the junction between the feeding line and the modulation electrode.
Since the resonance type optical modulator according to any one of the first to fourth points of the present invention thus configured can use an open-ended modulation electrode, the fifth point of the present invention relates to a modulator wherein the common electrode formed in opposition to the modulation electrode is open-ended at both ends, in addition to any one of the first to fourth points.
Since the resonance type optical modulator according to any one of the first to fourth points of the present invention thus configured can use a short-ended modulation electrode, the sixth point of the present invention relates to a modulator wherein the common electrode formed in opposition to the modulation electrode is short-ended at both ends, in addition to any one of the first to fourth points.
The stub used can be an open-ended stub or a short-ended stub. Therefore, the seventh point of the present invention relates to a modulator in which the stub is an open-ended stub in addition to any one of the first to sixth points, and the eighth point of the present invention relates to a modulator in which the stub is a short-ended stub.
A resonance type optical modulator having an asymmetric electrode has the following features. The ninth point of the present invention relates to a resonance type having an asymmetric electrode, comprising an optical path having electro-optical effect characteristics, an open stub, a short stub connected to the open stub, a feeder line that is electromagnetically connected to the open stub and the short stub, and a common electrode; the open stub and the short stub being formed on a single optical path and comprising a modulation electrode for applying an electric field to the optical path.
The tenth point of the present invention relates to a modulator including an optical path having electro-optical effect characteristics, a first short stub, a second short stub having a different length from the first short stub that is connected to the first short stub, a feeding line that is electromagnetically connected to the first short stub and the second short stub, and a common electrode; the first short stub and the second short stub being formed on a single optical path and comprising a modulation electrode for applying an electric field to the optical path.
The eleventh point of the present invention relates to a modulator including an optical path having electro-optical effect characteristics, a first open stub, a second open stub having a different length from the first open stub that is connected to the first open stub, a feeding line that is electromagnetically connected to the first open stub and the second open stub, and a common electrode; the first open stub and the second open stub being formed on a single optical path and comprising a modulation electrode for applying an electric field to the optical path.